Anaerobic Baffled Reactor - A Review

REVIEW PAPER
THE USE OF THE ANAEROBIC BAFFLED REACTOR
(ABR) FOR WASTEWATER TREATMENT: A REVIEW
WILLIAM P. BARBER*M
and DAVID C. STUCKEY**M
Department of Chemical Engineering and Chemical Technology, Imperial College of Science,
Technology and Medicine, Prince Consort Road, London SW7 2BY, U.K.
(First received May 1998; accepted in revised form August 1998)
AbstractÐA review concerning the development, applicability and possible future application of the an-
aerobic ba‚ed reactor for wastewater treatment is presented. The reactor design has been developed
since the early 1980s and has several advantages over well established systems such as the up¯ow an-
aerobic sludge blanket and the anaerobic ®lter. These include: better resilience to hydraulic and organic
shock loadings, longer biomass retention times, lower sludge yields, and the ability to partially separate
between the various phases of anaerobic catabolism. The latter causes a shift in bacterial populations
allowing increased protection against toxic materials and higher resistance to changes in environmental
parameters such as pH and temperature. The physical structure of the anaerobic ba‚ed reactor enables
important modi®cations to be made such as the insertion of an aerobic polishing stage, resulting in a
reactor which is capable of treating dicult wastewaters which currently require several units, ulti-
mately signi®cantly reducing capital costs. # 1999 Elsevier Science Ltd. All rights reserved
Key wordsÐanaerobic ba‚ed reactor, anaerobic digestion, reactor development, performance, solids
retention, molids odelling, full-scale.
INTRODUCTION
The successful application of anaerobic technology
to the treatment of industrial wastewaters is criti-
cally dependent on the development, and use, of
high rate anaerobic bioreactors. These reactors
achieve a high reaction rate per unit reactor volume
(in terms of kg COD/m3
d) by retaining the biomass
(Solids Retention Time, SRT) in the reactor inde-
pendently of the incoming wastewater (Hydraulic
Residence Time, HRT), in contrast to Continually
Stirred Tank Reactors (CSTRs), thus reducing reac-
tor volume and ultimately allowing the application
of high volumetric loading rates, e.g. 10±40 kg
COD/m3
d (Iza et al., 1991). High rate anaerobic
biological reactors may be classi®ed into three
broad groups depending on the mechanism used to
achieve biomass detention, and these are ®xed ®lm,
suspended growth, and hybrid. There are currently
900 full-scale installations in the world today
(Habets, 1996), and they are distributed as follows:
Up¯ow Anaerobic Sludge Blanket (UASB±sus-
pended growth) 67% (Lettinga et al., 1980); CSTR
12%; Anaerobic Filter (AF±®xed ®lm) 7% (Young
and McCarty, 1969); other 14%. The highest load-
ing rates achieved during anaerobic treatment to
date are attributed to the ``Anaerobic Attached
Film Expanded Bed'' (AAFEB) reactor (120 kg
COD/m3
d, Switzenbaum and Jewell (1980)), but its
inherent complexity and high operating costs limit
its practical use on a wide scale.
Around the same time as Lettinga developed the
UASB, McCarty and co-workers at Stanford
noticed that most of the biomass present within an
anaerobic Rotating Biological Contactor (RBC,
Tait and Freidman (1980)) was actually suspended,
and when they removed the rotating discs they
developed the Anaerobic Ba‚ed Reactor (ABR,
McCarty (1981)). However, ba‚ed reactor units
had previously been used to generate a methane
rich biogas as an energy source (Chynoweth et al.,
1980). Although not commonly found on a large
scale, the ABR has several advantages over other
well established systems, and these are summarised
in Table 1.
Probably the most signi®cant advantage of the
ABR is its ability to separate acidogenesis and
methanogenesis longitudinally down the reactor,
allowing the reactor to behave as a two-phase sys-
tem without the associated control problems and
high costs (Weiland and Rozzi, 1991). Two-phase
operation can increase acidogenic and methano-
genic activity by a factor of up to four as acido-
genic bacteria accumulate within the ®rst stage
Wat. Res. Vol. 33, No. 7, pp. 1559±1578, 1999
# 1999 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0043-1354/99/$ - see front matterPII: S0043-1354(98)00371-6
*Author to whom all correspondence should be addressed.
[Tel. +44-171-594-5591; Fax: +44-171-594-5604, E-
mail: d.stuckey@ic.ac.uk].
1559

(Cohen et al., 1980, 1982), and di€erent bacterial
groups can develop under more favourable con-
ditions. The advantages of two-phase operation
have been extensively documented (Pohland and
Ghosh, 1971; Ghosh et al., 1975; Cohen et al.,
1980, 1982). These bene®ts have catalysed the devel-
opment of other staged reactor con®gurations such
as the ``Multiplate Anaerobic Reactor'' (El-
Mamouni et al., 1992; Guiot et al., 1995), ``Up¯ow
Staged Sludge Bed (USSB)'' (van Lier et al., 1994,
1996) and the ``Staged Anaerobic Filter'' (Alves et
all., 1997), all of which have showed considerable
potential for wastewater treatment. Disadvantages
of the ba‚ed reactor design at pilot/full-scale
include the requirement to build shallow reactors to
maintain acceptable liquid and gas up¯ow vel-
ocities, and problems with maintaining an even dis-
tribution of the in¯uent (Tilche and Vieira, 1991).
However, despite its many potential advantages
over other high rate anaerobic reactor designs, and
the ever-increasing number of publications, there
has never been any attempt to collate all this infor-
mation in a review. Hence, the objective of this
paper is to review the currently available literature
on the ABR, focusing on reactor development, hy-
drodynamics, performance, biomass characteristics
and retention, modelling, full-scale operation and a
comparison with other well established alternatives.
Finally, based on the review, a closing section will
discuss future prospects for the ABR.
REACTOR DEVELOPMENT
The ABR is a reactor design which uses a series
of ba‚es to force a wastewater containing organic
pollutants to ¯ow under and over (or through) the
ba‚es as it passes from the inlet to the outlet
(McCarty and Bachmann, 1992). Bacteria within
the reactor gently rise and settle due to ¯ow charac-
teristics and gas production, but move down the
reactor at a slow rate. The original design is shown
in Fig. 1(C), although Fig. 1(A) is more commonly
recognised. However, in order to improve reactor
performance several modi®cations have been made
(Fig. 1(B and D±J)). The main driving force behind
reactor design has been to enhance the solids reten-
tion capacity, but other modi®cations have been
made in order to treat dicult wastewaters (e.g.
with a high solids content, Boopathy and Sievers
(1991)), or simply to reduce capital costs (Orozco
(1997), Fig. 1(F)). A summary of the main altera-
tions is shown in Table 2.
In 1981, Fannin et al. (1981) added vertical baf-
¯es to a plug-¯ow reactor treating high solids sea
kelp slurry (Fig. 1(C)) in order to enhance the reac-
tor's ability to maintain high populations of slowly
growing methanogens, which were being replaced
by the in¯uent solids. With a constant loading rate
of 1.6 kg COD/m3
d methane levels increased from
30 to over 55% with a methane yield of 0.34 m3
/kg
VSS after the ba‚es were added. In a later study,
Bachmann et al. (1983) compared the performance
of two ba‚ed reactors before and after narrowing
the down¯ow chambers and slanting the ba‚e
edges (Fig. 1(A) and Table 2). Although methane
production rates and reactor eciency were
improved in the modi®ed design, a decrease in the
methane content of the biogas was also noted.
Despite the alterations, the performance of both
reactors was inferior to an anaerobic ®lter and
rotating biological disc operated under the same
conditions. COD removal eciencies were 82, 92
and 90% for the modi®ed ba‚e, anaerobic ®lter,
and rotating biological disc reactors respectively.
The next major change occurred with the devel-
opment of the ®rst of several hybrid designs (Tilche
and Yang, 1987, Fig. 1(E)). The motivation behind
the alterations was based on enhancing solids reten-
tion for high strength wastewater treatment. The
reactor was signi®cantly larger than those used pre-
viously, and incorporated a solids settling chamber
after the ®nal compartment. Solids washed out
from the ba‚ed reactor were collected in the
settling chamber and subsequently recycled to the
®rst compartment. Packing was also positioned at
the liquid surface of each compartment with ran-
domly packed Pall rings in the ®rst two chambers,
and a deeper, structured, modular corrugated block
which had a high voidage in the third chamber.
Bio¯ocs, which became buoyant due to a reduction
in density caused by high gas production, were
retained in the ®rst chamber due to the packing.
Higher loading rates were possible with this struc-
ture due to minimal solids washout during elevated
gas mixing. Each gas chamber was separated per-
mitting the measurement of gas composition and
production from each compartment. Although ben-
e®cial in this regard, the separation of the gas can
Table 1. Advantages associated with the anaerobic ba‚ed reactor
Advantage
Construction
1 Simple design
2 No moving parts
3 No mechanical mixing
4 Inexpensive to construct
5 High void volume
6 Reduced clogging
7 Reduced sludge bed expansion
8 Low capital and operating costs
Biomass
1 No requirement for biomass with unusual settling properties
2 Low sludge generation
3 High solids retention times
4 Retention of biomass without ®xed media or a solid-settling
chamber
5 No special gas or sludge separation required
Operation
1 Low HRT
2 Intermittent operation possible
3 Extremely stable to hydraulic shock loads
4 Protection from toxic materials in in¯uent
5 Long operation times without sludge wasting
6 High stability to organic shocks
William P. Barber and David C. Stuckey1560

also enhance reactor stability by shielding syn-
trophic bacteria from the elevated levels of hydro-
gen which are found in the front compartments of
the ba‚ed reactor.
In order to treat swine wastewater containing a
high content of small particulate material,
Boopathy and Sievers (1991) further modi®ed the
ba‚ed reactor. The main problems associated with
the treatment of swine wastewater in a ba‚ed reac-
tor were the inability to produce a ¯oating sludge
layer which would enhance solids retention, and,
the high velocities associated with the ba‚es caused
signi®cant washout of solid material. Therefore, the
ba‚ed reactor was modi®ed to reduce up¯ow liquid
velocities and to accept whole waste. The ®rst com-
partment of a two-chamber unit was doubled in
size to 10 l and this was followed by a second com-
partment of 5 l (Fig. 1(G)). Performance character-
istics and solids retention capabilities were
compared with a three-chamber unit with equal
volume chambers. The additional chamber in the
three-compartment unit, together with physical
modi®cations, provided a longer solids retention
time and superior performance than the reactor
with only two compartments. This was in contrast
to earlier ®ndings (Sievers, 1988), when no di€er-
ence was found in treatment eciency compared
with compartment number in unmodi®ed reactors.
Fig. 1. Variations of the ba‚ed reactor. (A) Single gas headspace, (B) individual gas headspace, (C)
vertical, (D) horizontal, (E) hybrid with settling zone, (F) open top, (G) enlarged ®rst compartment,
(H±J) various packing arrangements: (H) up-comers, (I) down-comers, (J) entire reactor. Key:
W = Wastewater, B = Biogas, E = E‚uent, S = Solids, (shaded areas represent random packing).
The anaerobic ba‚ed reactor: a review 1561

The larger compartment in the two-compartment
reactor acted as a natural ®lter and provided su-
perior solids retention for the small particles. The
reactor collected twice the amount of solid material
(20.9 g/l) than the reactor with three chambers. This
was further substantiated in the solids washout
data, which was lower in the two-compartment
reactor despite showing lower treatment eciency.
Further analysis showed that despite losing more
solids, the three-compartment reactor was more e-
cient at converting the trapped solids to methane.
REACTOR HYDRODYNAMICS
Flow patterns
The hydrodynamics and degree of mixing that
occur within a reactor of this design strongly in¯u-
ence the extent of contact between substrate and
bacteria, thus controlling mass transfer and poten-
tial reactor performance. In 1992, Grobicki and
Stuckey conducted a series of residence time distri-
bution studies by tracking the fate of an inert tracer
(Li+
) in the e‚uent of a number of ba‚ed reactors
(4±8 chambers), both with and without biomass, at
various HRTs, and incorporated the data into
``Dispersion'' and ``Tanks In Series'' models pre-
viously described by Levenspiel (1972). The models
provided a useful method to calculate the degree of
mixing and the amount of unused volume (known
as ``dead space'') within the reactor. They found
low levels of dead space (<8% hydraulic dead
space in an empty reactor) in comparison with
other anaerobic reactor designs, e.g. 50±93% in an
anaerobic ®lter (Young and Young, 1988), and
>80% in a CSTR (Stuckey, 1983).
Dead space increased to 18% on the addition of
8 g VSS/l, however, no direct correlation between
hydraulic dead space and HRT could be drawn. At
low HRT, the presence of biomass had no signi®-
cant e€ect on hydraulic dead space, which was
found to be a function of ¯owrate and number of
ba‚es. This contrasted with biological dead space,
which was found to be a function of biomass con-
centration, gas production, and ¯owrate, and which
increased with increasing ¯owrates. At high loading
rates caused by low HRT, gas production as well as
increased ¯owrates kept sludge beds partly ¯uidised.
Therefore, the contradictory e€ects of hydraulic
and biological dead space prevented a correlation
being derived between HRT and overall dead space.
Biological dead space was established as the major
contributor to overall dead space at high HRT, but
its e€ect decreased at lower HRT since gas pro-
duction disrupted channelling within the biomass
bed. Severe channelling, caused by large hydraulic
shocks, was found to be bene®cial since most of the
biomass was not entrained in the ¯ow, and this
resulted in low washout and a fast recovery in per-
formance (Grobicki and Stuckey, 1992; Nachaiyasit
and Stuckey, 1997c). Nevertheless, investigations of
the hydrodynamics to date have not taken into
account various other factors which are probably
important, and these include biogas mixing e€ects,
viscosity changes due to extracellular polymer pro-
duction, and biomass particle size. In addition, no
work has been done of the rate at which solid par-
ticles/biomass move down the reactor.
E€ect of e‚uent recycle
Recycling the e‚uent stream tends to reduce
removal eciency because the reactor approaches a
completely mixed system, and therefore the mass
transfer driving force for substrate removal
decreases despite a small increase in the loading
rate. The e€ect of loading rate and increasing re-
cycle ratios on performance is shown in Table 3.
Chynoweth et al. (1980) observed a positive e€ect
caused by recycling twenty percent of the e‚uent,
when the methane yield increased by over 30%.
The addition of a recycle stream was also found to
alleviate the problems of low pH caused by high
levels of volatile acids at the front of the reactor,
and discourage gelatinous bacterial growth at the
reactor inlet for the treatment of a complex protein
carbohydrate wastewater (Bachmann et al., 1983).
Another bene®t of recycle is the dilution of toxi-
cants and reduction of substrate inhibition in the
Table 2. Development of the ABR
Fig. Modi®cation Purpose Ref.
1(C) addition of vertical ba‚es to a plug-
¯ow reactor
enhances solids retention to allow
better substrate accessibility to
methanogens
Fannin et al., 1981
1(A) (i) down¯ow chambers narrowed (i) encourages cell retention in up¯ow
chambers
Bachmann et al., 1983
(ii) slanted edges on ba‚es (40±458) (ii) routes ¯ow towards centre of
compartment encouraging mixing
1(E) (i) settling chamber (i) enhances solids retention Tilche and Yang, 1987
(ii) packing positioned at top of each
chamber
(ii) prevents washout of solids
(iii) separated gas chambers (iii) ease and control of gas
measurement, provides enhanced
reactor stability
1(G) enlargement of ®rst chamber better treatability of high solids
wastewater
Boopathy and Sievers, 1991

in¯uent. (Bachmann et al., 1985; Grobicki and
Stuckey, 1991).
From theoretical considerations, recycle should
have a negative e€ect on reactor hydrodynamics by
causing increased mixing (which encourages solids
loss, and disrupts microstructures of bacteria living
in symbiotic relationships (Henze and HarremoeÈ s,
1983)) and enhancing the amount of dead space
(Grobicki and Stuckey, 1992; Nachaiyasit, 1995). In
her thesis, Nachaiyasit (1995) showed that dead
space doubled to approximately 40% when the re-
cycle ratio was increased from zero to 2. The author
also reported a sudden loss of solids when the re-
cycle ratio was doubled. Increasing recycle has also
been linked to an increase in the sludge volume
index using anaerobic ®lters (Matsushige et al.,
1990).
Mixing caused by recycle has also been found to
cause a return to single phase digestion, therefore
the bene®ts arising from the separation of acido-
genic and methanogenic phases are partially lost.
Bachmann et al. (1985) noticed that methanogenic
activity was more uniformly distributed over the
whole reactor after recycle was used. The conse-
quences of this observation are scavenging bacteria
(such as Methanosaeta) will end up at the front of
the reactor where harsh conditions of high substrate
concentration, high hydrogen partial pressure and
low pH will make them relatively inactive, and
poorly scavenging acid producing bacteria pushed
towards the rear of the reactor will be starved since
less substrate will be available. Nachaiyasit (1995)
discovered a fall in both gas production and
methane composition down the reactor when the re-
cycle ratio was increased.
The overall bene®ts of recycle are unclear, and ul-
timately its use will depend on the type of waste
being treated. If pH problems are severe, the in¯u-
ent has high levels of toxic material, or high loading
rates are preferred then recycle will be bene®cial.
However, as can be seen in Table 4, the disadvan-
tages of recycle show that it should be used with
caution, and only when absolutely necessary.
REACTOR PERFORMANCE
Start-up
The overall objective of start-up is the develop-
ment of the most appropriate microbial culture for
the waste stream in question. Once the biomass has
been established, either as a granular particle or a
¯oc, reactor operation is quite stable. The import-
ant factors governing the start-up of anaerobic reac-
tors have been summarised in the literature
(Stronach et al., 1986; Weiland and Rozzi, 1991;
Hickey et al., 1991), and will not be discussed here.
A collection of data obtained during reactor start-
up is shown in Table 5.
Initial loading rates should be low so that slow
growing micro-organisms are not overloaded, and
both gas and liquid up¯ow velocities should be low
so that ¯occulent and granular growth is encour-
aged. The recommended initial loading rate is ap-
Table 3. Reactor performance vs increasing recycle ratio
Recycle ratio Reactor volume (l) In¯uent COD (g/l) Organic loading
rate (kg/m3
d)
COD removal (%) Ref.
0 13 8 2.70 93b
Bachmann et al., 1985a
0 10 4 4.80 99 Nachaiyasit and Stuckey, 1995b
0.1 10 4 4.80 98 Nachaiyasit and Stuckey, 1995b
0.5 13 8 2.86 88c
2.2 13 8 3.85 81c
3 13 8 3.42 91 Bachmann et al., 1985a
5 13 8 5.76 77 Bachmann et al., 1985a
6 13 8 6.83 75 Bachmann et al., 1985a
9.6 13 8 11.01 68 Bachmann et al., 1985a
11.7 13 8 16.92 55 Bachmann et al., 1985a
13.8 13 8 17.62 60 Bachmann et al., 1985a
a
Recycle ratios calculated from data supplied based on R = 0 for retention time of 71 h, organic loading rates converted from hydraulic
loading rates supplied.b
Loading rates calculated from recycle ratio data.c
Nutrient limited conditions.
Table 4. Advantages and disadvantages of e‚uent recycle
Advantages Disadvantages
1 Front pH increased 1 Overall eciency reduced
2 Reduction of in¯uent toxicity and substrate
inhibition
2 Increased solids loss
3 Higher loading rates possible 3 Increased hydraulic dead space
4 Better substrate/biomass contact 4 Disruption of bacterial communities and bio¯ocs
5 Encourages one-phase digestion

proximately 1.2 kg COD/m3
d (Henze and
HarremoeÈ s, 1983), however, successful start-up of a
pilot scale ABR has been achieved at signi®cantly
higher primary loading rates (Table 5, Boopathy
and Tilche (1991)). Although Nachaiyasit (1995)
originally noted adequate performance with an in-
itial loading rate of 13 kg COD/m3
d, an accumu-
lation of intermediate products caused reactor
souring and eventual failure after two weeks of oper-
ation. A possible way to prevent failure by overload-
ing was employed in 1980 by Chynoweth and co-
workers. In order to stimulate the growth of methano-
genic archea, pulses of methane precursors (acetate
and/or an acetate/formate mixture) were added
directly before raising loading rates, and these were
e€ective in minimising the shock caused by a sudden
increase in organic loading. Alternative methods to
prevent failure include the adjustment of pH in the
®rst compartment (Grobicki, 1989). A recent study
(Barber and Stuckey, 1997) has shown that maintain-
ing an initially long detention time (80 h) which is
reduced in a stepwise fashion during which time sub-
strate concentration is kept constant, provides greater
reactor stability and superior performance than a
reactor started-up with a constant and low detention
time coupled to a stepwise increase in substrate con-
centration. These ®ndings were linked to better solids
accumulation, promotion of methanogenic popu-
lations, and faster recovery to hydraulic shocks in the
reactor started at the longer retention time.
Treatment applications
This section reviews the performance of the
ba‚ed reactor while treating a variety of waste-
waters, in particular, low and high strength, low
temperature, high in¯uent solids and sulphate con-
taining waste. Tables 6 and 7 and Fig. 2 summarise
the available literature.
Low strength treatment. Various authors have
treated low strength wastewaters e€ectively in the
ABR, as shown in Table 8. Dilute wastewaters
inherently provide a low mass transfer driving force
between biomass and substrate, and subsequently
biomass activities will be greatly reduced according
to Monod kinetics. As a result, treatment of low
strength wastewaters has been found to encourage
the dominance of scavenging bacteria such as
Methanosaeta in the ABR (Polprasert et al., 1992).
Hassouna and Stuckey (1998), have shown that no
substantial change occurred in the population of
acid producing bacteria down the length of a reac-
tor treating dilute milk waste, indicating the lack of
signi®cant population selection at low COD concen-
trations.
It appears that biomass retention is enhanced sig-
ni®cantly due to lower gas production rates
suggesting that low hydraulic retention times
(6 4 2 h) are feasible during low strength treat-
ment. Orozco (1988) noted decreasing overall gas
production with increasing HRTs, and this implied
possible biomass starvation in later compartments
at longer retention times. Another important conse-
quence of low retention times when treating dilute
wastewaters is an increase in hydraulic turbulence,
which can lower apparent Ks values (Kato et al.,
1997) thus enhancing treatment eciency.
Witthauer and Stuckey (1982) observed irregular
COD removal in ba‚ed reactors run at low loading
rates and long retention times when treating dilute
synthetic greywater. These problems were associated
with low sludge blankets (inoculum contained less
than 3 g VSS/l) caused after long periods of biomass
settling. Channels were formed within the low blan-
kets and this resulted in low gas productivity in most
of the sludge blanket except for around the channels.
Hence, biogas mixing was greatly reduced and this
resulted in minimal biomass/substrate mass transport.
In contrast, anaerobic ®lters, operated under the same
conditions, outperformed the ba‚ed reactors, even
after their suspended biomass was ¯ushed out in a
hydraulic shock experiment. The authors rec-
ommended that when treating dilute wastewater,
ba‚ed reactors should be started-up with higher bio-
mass concentrations (than used in their study) in
order to obtain a suciently high sludge blanket (and
better gas mixing) in as short a time as possible.
Table 5. Start-up data for the ABR
LRa
initial
Timeb
initial
LR
LR
increased
Time
increased LR
LR
®nal
Start-up
timec
(d)
Initial
VSS (g/l)
Ref.
1 (ramp increase) 4 57 NGd
Boopathy and Sievers, 1991
2 (ramp increase) 20 >60 NG Bachmann et al., 1983
0.4 NG 0.53 NG 0.8 >60 NG Yang and Moengangongo, 1987
4.33 40 10.26 22 12.25 62 4.01 Boopathy and Tilche, 1991
1.2 7 2.4 10 4.8 77 8.77 Grobicki, 1989
0.97 NG NG NG 12.25 78 4.01 Boopathy and Tilche, 1992
2.2 90 2.6 135 3.5 90 NG Boopathy et al., 1988
13.04 failed À À À 18 Nachaiyasit, 1995
4.35 failed À À À 18 Nachaiyasit, 1995
1.2 NG 2.4 NG 4.8 >95 18 Nachaiyasit, 1995
NG (ramp increase) 20 >100 NG Fox and Venkatasubbiah, 1996
1.2 53 2.4 24 4.8 128 18 Barber and Stuckey, 1997
a
LR = loading rate in kg COD/m3
d.b
The amount of time spent at each loading rate (d).c
Start-up time quoted is the time required for
reactor to reach steady state.d
NG = data not given.

Table6.Performancedataonba‚edreactorsystemsa
SubstrateVolume(l)ChambersBiomass(gVSS/l)InletCOD(mgCOD/l)Loadingrate(kg/m3
/d)CODremoval(%)HRT(h)Temperature(8C)Ref.
Undilutedseakelp9.856000±36,0000.4±2.436035Chynowethetal.,1980
Dilutedseakelp1041.635Fanninetal.,1981,1982
10467,200±89,6005.6±6.4288±33635
10480,0001.6120035
Carbohydrate±protein6.357100±76002±2079±8235Bachmannetal.,1983
Syntheticgreywater864800.1±0.463±8448±8425±33WitthauerandStuckey,1982
Carbohydrate±protein6.3580002.5±3655±934.8±7135Bachmannetal.,1985
Dilutedswinewastewater20À<50001.8756030YangandMoengangongo,1987
Molasseswastewater15035.35000±10,0005.59837Yangetal.,1988
Sucrose7511344±5000.7±285±936±1213±16Orozco,1988
Whiskydistillerywastewater6.3551,6002.2±3.59036030Boopathyetal.,1988
Carbohydrate±protein10840001.2±4.8b
992035GrobickiandStuckey,1989
Carbohydrate±protein7.8±10.44±83040001.2±4.89520±8035GrobickiandStuckey,1991
Molasseswastewater15034.01115,771±990,0004.3±2849±88138±85037BoopathyandTilche,1991
Molasseswastewater15034.01115,771±990,0002070H13837BoopathyandTilche,1992
Swinemanure152±358,500462±6936035BoopathyandSievers,1991
Municipalwastewater3503264±9062.17904.8±1518±28Garutietal.,1992
Slaughterhousewastewater5.164450±5500.9±4.7375±902.5±2625±30Polprasertetal.,1992
Carbohydrate±protein104±80±8.540001±8035GrobickiandStuckey,1992
Molasseswastewater1503115,771±990,0001040±7524±14437XingandTilche,1992
Molasseswastewater15034.11and7.21115,771±990,00020>70H14037Xingetal.,1991
Carbohydrate±protein1081840001.2±4.898,9320,8035NachaiyasitandStuckey,1995
Pharmaceuticalwastewater10520,0002036±682435FoxandVenkatasubbiah,1996
Phenolic520±252200±31921.67±2.583±94H2421Holtetal.,1997
Glucose651000±10,0002±2072±991235Baeetal.,1997
Carbohydrate±protein108181000±40001.2±4.89820±8035BarberandStuckey,1998
Domesticsewage/industrialwaste394,0008315c
0.857010.315Orozco,1997
Carbohydrate±protein1081840001.2±4.875±83,93±97,9620,20,2015,25,35NachaiyasitandStuckey,1997a
Carbohydrate±protein1081840004.8±9.690±982035NachaiyasitandStuckey,1997b
Carbohydrate±protein1081840004.8±1852±981±2035NachaiyasitandStuckey,1997c
a
Containscalculatedresults,eitherfromgraphsorfromsupplieddata.b
Alsowithshockloadingof96kg/m3
d.c
BOD5value.

High strength treatment. Whereas low retention
times are possible and even necessary for dilute
wastewaters, the opposite applies when treated con-
centrated waste. This is mainly due to the high gas
mixing caused by improved mass transfer between
the biomass and substrate. This will result in high
biomass wastage, and has led to modi®cations in
the reactor design in order to enhance solids reten-
tion (see Section 2). A brief summary of the litera-
ture available on high strength treatment is shown
in Table 9.
When Boopathy and Tilche (1991) changed the
in¯uent to a 150 l hybrid reactor from 115 g
COD/l molasses alcohol stillage with a loading
rate of 12.25 kg/m3
d to raw alcohol molasses
(990 g COD/l, OLR = 28 kg COD/m3
d) they
noticed an increase in overall gas production of
over 65% within 3 weeks, a drop in COD
removal of 20%, a fall in the methane compo-
sition of the biogas by 20% for one week which
then recovered (implying initial overloading of
methanogens), and an approximate increase in
volatile suspended solids of 50% within 3 weeks.
Higher levels of gas production increased sludge
bed expansion, but the improved settling ability
of the biomass may have reduced the e€ects of
solids loss caused by the gas (Boopathy and
Tilche, 1991). This observation was partially con-
®rmed in an earlier study (Boopathy et al., 1988)
where no increase in solids loss or decrease in
performance were noted when loading rates were
increased from 2.6 to 3.5 kg COD/m3
d. However,
minimal solids were lost to the e‚uent at equally
low loading rates in the work by Boopathy and
Tilche (1991), but levels increased to 17 g VSS/l
at higher loading rates. (The reactor contained
approximately 1.25 g VSS/l of reactor in a 150 l
volume.) According to kinetic considerations, high
substrate concentrations will encourage both fast
growing bacteria, and organisms with high Ks
values, and methane production will be derived
mainly from acetate decarboxylation by
Table 7. Potential methane yields from ba‚ed reactors
Wastewater OLR (kg/m3
d) Methane yield (m3
/kg VSS/d) Ref.
Swine manure 4±8 0.76±1.28 Boopathy and Sievers, 1991
Swine manure 1.8 0.27 Yang and Moengangongo, 1987
Carbohydrate/protein 4.8 0.11 Nachaiyasit and Stuckey, 1995
Carbohydrate/protein 4.8 0.22 Grobicki, 1989
Sea kelp 2.4 0.35 Chynoweth et al., 1980
Molasses 20 1.25 Boopathy and Tilche, 1991
Phenol 1.67±2.5 0.26±0.34 Holt et al., 1997
Slaughterhouse 1.82±4.73 0.13±0.18 Polprasert et al., 1992
Fig. 2. Performance eciency against various loading rates.

Methanosarcina sp. and hydrogen scavenging
methanogens (such as Methanobrevibacter and
Methanobacterium). Subsequently Methanosarcina
sp. was observed as the dominant bacterial species
in bio¯ocs formed during high strength treatment
(Boopathy and Tilche, 1991). (See Section 5.2.)
Low temperature treatment. At low/ambient tem-
peratures van Lier et al. (1996), found signi®cant
advantages with respect to reactor performance for
staged reactors when compared with completely
mixed systems. From Table 6 it can be seen that
the vast majority of work done so far on the ba‚ed
reactor has been conducted in the mesophilic tem-
perature range. However, the ba‚ed reactor has
been run as low as 138C (Orozco, 1988), although
the most extensive study at low temperatures in the
ba‚ed reactor was carried out by Nachaiyasit and
Stuckey (1997a, Table 10).
Generally, biochemical reactions double in rela-
tive activity for every 108C increase in temperature
in accordance with the van `t Ho€ rule over a
restricted temperature range. In spite of this,
Nachaiyasit (1995), found no signi®cant reduction
in overall COD removal eciency when the tem-
perature of an ABR was dropped from 35 to 258C,
with steady state reached after only two weeks.
However, lower catabolic rates caused by elevated
Ks values (according to Arrhenius kinetics) at the
front of the reactor caused a shift in acid pro-
duction towards the rear, although overall removal
was una€ected. An increase in VFA production
caused a simultaneous reduction in pH and an in-
itial increase in gas phase hydrogen that quickly
returned to below background levels. The deeper
penetration of the VFAs down the reactor should
potentially improve the growth yields of the metha-
nogens in the latter compartments. The results
showed that slower growing organisms exhibited a
greater sensitivity to a fall in temperature compared
to bacteria with faster growth kinetics, and this is
in accordance with literature ®ndings (Cayless et
al., 1989; Kotsyurbenko et al., 1993; Borja et al.,
1994; Speece, 1996). Similar high treatment ecien-
cies at ambient temperature have also been noted
for a medium strength phenolic wastewater (Holt et
al., 1997).
Nachaiyasit and Stuckey (1997a) further reduced
the temperature to 158C, and a fall in overall e-
ciency of 20% was noted after one month. Changes
in performance down the reactor occurred over a
long period of time in contrast to CSTRs. This is
advantageous since the slow response would inher-
ently provide more protection to shocks than in
other reactor systems. However, despite the fact
that the reactors were kept for long periods of time
at reduced temperatures (12 weeks) their perform-
ance did not improve despite the increased inter-
mediate acid concentrations, which according to
Monod kinetics should encourage more biomass
growth to compensate for the increased substrate
levels. This may be due to the fact that Ks increases
substantially as temperature falls, (Lawrence and
McCarty, 1969) leaving low levels of VFAs that
cannot be degraded.
This study also found that the fraction of VFAs
in the e‚uent in terms of COD had reduced signi®-
cantly. VFAs contributed to approximately a third
of the COD at 158C, and two thirds at 258C, indi-
cating that the production of refractory material
(termed as Soluble Microbial Products (SMPs),
Table 8. Selected low strength performance data
Wastewater HRT (h) COD (mg/l) COD removal (%) OLR (kg/m3
/d) Gas produced (v/v/d) Ref.
in¯uent e‚uent
Greywater 84 438 109 75 0.13 0.025 Witthauer and Stuckey, 1982
Greywater 48 492 143 71 0.25 0.05 Witthauer and Stuckey, 1982
Greywatera
84 445 72 84 0.13 0.025 Witthauer and Stuckey, 1982
Sucroseb
6.8 473 74 74 1.67 0.49 Orozco, 1988
Sucroseb
8 473 66 86 1.42 0.43 Orozco, 1988
Sucroseb
11 441 33 93 0.96 0.31 Orozco, 1988
Slaughterhouse 26.4 730 80 89 0.67 0.72 Polprasert et al., 1992
a
Temperature at 258C.b
Temperatures lower than 168C. All other work shown in table performed in a mesophilic temperature range
Table 9. Selected high strength treatment data
Wastewater Raw molasses Molasses alcohol stillage Swine waste Whisky distillery
In¯uent COD (g/l) 990 115.8 58.5 51
HRT (h) 850 138±636 360 360
Reactor volume (l) 150 150 15 6.3
Temperature (8C) 37 37 35 30
OLR (kg/m3
d) 28 4.3±20 4 2.2±3.46
COD removal (%) 50 70±88 62±69 >90
Biogas production (v/v/d) >5 >2.3 2.9±3.2 1.2±3.6
Ref. Boopathy and Tilche, 1991 Boopathy and Tilche, 1992 Boopathy and Sievers, 1991 Boopathy et al., 1988

Rittmann et al. (1987)) increased substantially at
lower temperatures. In conclusion, the work found
that a combination of decreased catabolic rates,
increased Ks, and higher levels of refractory ma-
terial caused inferior performance at 158C, but that
a drop in temperature from 35 to 258C had negli-
gible e€ects on overall reactor performance despite
predictions from the van `t Ho€ rule. This has been
observed before in bio®lm/¯oc based reactors where
mass transfer limited biomass activity (Hickey et
al., 1987). However, Nachaiyasit's work did not
consider the e€ects of nutrient (especially iron)
bioavailability, which may be reduced at lower tem-
peratures (Speece, 1996), nor did it investigate the
signi®cance of temperature on ionisation equilibria
which inevitably controls the potential toxicity of
materials, some of which may be tolerated at higher
temperatures (Sawyer et al., 1994).
High solids treatment. In early work, Chyno-
weth's group in Illinois (1980, 1981) used ba‚ed
reactors to generate methane from sea kelp as an
alternative energy source. Although the COD of the
kelp was not quoted, the feed contained 15% total
solids, which were ground and chopped. Practical
problems associated with feeding solids were over-
come by applying the substrate by syringe. During
a particular run, signi®cant solids build-up was
observed in the ®rst compartment after 2 weeks of
operation. The solids build-up reduced micro-
organism contact with the substrate therefore mini-
mising hydrolysis and subsequent bioconversion.
Performance was signi®cantly improved after manu-
ally agitating the reactor for a short time period.
Solid material was also found to physically displace
biomass within the reactor indicating that modi®-
cations to the ABR would be required for high
solids treatment.
In 1991, Boopathy and Sievers modi®ed the
ba‚ed reactor (see Section 2) to treat high strength
swine waste (see Table 6) containing 51.7 g/l total
solids. When a loading rate of 4 kg COD/m3
d with
a retention time of 15 d was applied, removal rates
for COD (70 and 80%), and total solids (60 and
74%) were achieved for two- and three-compart-
ment reactors respectively. Solids retention times
were experimentally determined to be over 20 d in
both reactors. The study found that the majority of
the protein fraction of the solids was retained
within the reactor, compared with a lower retention
of cellulose/hemicellulose, and a virtual loss of all
lipid material, although the authors o€ered no ex-
planation to the cause. Previous work in the same
laboratory had shown protein to be dicult to
degrade but a great potential source of methane,
hence its detention proved to be signi®cant in reac-
tor performance.
Sulphate treatment. Fox and Venkatasubbiah
(1996), investigated the e€ects of sulphate reduction
in the ABR by treating a sulphate containing phar-
maceutical wastewater up to a ®nal strength of 20 g
COD/l with a COD:SO4 ratio of 8:1. At steady
state, 50% COD removal and 95% sulphate re-
duction was possible with a detention time of 1 day.
Reactor pro®les showed that sulphate was almost
completely reduced to sulphide within the ®rst
chamber, and a concomitant increase in sulphide
levels down the reactor indicated that sulphate
redirected electron equivalents to hydrogen sulphide
in preference to methane.
After altering the COD:SO4 ratio by adding glu-
cose, isopropanol and sulphate, the authors noted a
fall in potential sulphate reduction from >95% at
COD:SO4=150:1 to <50% at COD:SO4=24:1.
Increasing sulphate concentrations with glucose and
isopropanol present showed inhibition of sulphate
reduction caused by elevated sulphide concen-
trations. Increasing the inlet concentration from 2
to 8 g COD/l (COD:SO4 at 8:1) over 100 d caused
an increase in the total e‚uent sulphide to toxic
levels (200 or 80 mg/l unionised H2S assuming pH
7, pH data not supplied), with COD removal drop-
ping to below 20%. VFA levels as high as 4500 mg/
l were observed during inhibition and these contrib-
uted to a maximum of 35% of the reactor e‚uent
COD. The major contributor to the e‚uent VFA
was acetate indicating inhibition of acetoclastic
methanogenesis and a distinct lack of acetate cleav-
ing sulphidogenesis. A recycle stream (recycle ratio
10:1) was employed to overcome sulphide inhibition
of both sulphate reducing bacteria and methano-
genic archea. The e‚uent was oxidised in a trickling
thin ®lm reactor in the presence of an enriched cul-
ture of Thiobacillus sp., which converted the sul-
phide to elemental sulphur. After employing
recycle, total e‚uent sulphide levels decreased to
below 75 mg/l (or 30 mg/l unionised H2S after pH
correction) with an increase in COD removal to
50%.
Table 10. Low temperature treatment
Temperature
(8C)
Inlet concentration
(mg COD/l)
Reactor
volume (l)
COD removed
(%)
Biogas
(v/v/d)
Relative
reaction rateb
Ref.
35 4000 10 96 2.78 1 Nachaiyasit and Stuckey, 1997a
25 4000 10 93±97 2.36 0.676 Nachaiyasit and Stuckey, 1997a
15 4000 10 75±83 1.74 0.391 Nachaiyasit and Stuckey, 1997a
13±16 500 75 84±92 0.31±0.50a
0.391 Orozco, 1988
a
Calculated from theoretical gas production based on COD removal.b
Reaction rate relative to that at 358C calculated from typical Q10
values for anaerobic processes (Sawyer et al., 1994).

BIOMASS CHARACTERISTICS AND RETENTION
CAPABILITIES
Bacterial populations
With the unique construction of the ABR various
pro®les of microbial communities may develop
within each compartment. The microbial ecology
within each reactor chamber will depend on the
type and amount of substrate present, as well as
external parameters such as pH and temperature. In
the acidi®cation zone of the ABR (front compart-
ment(s) of reactor) fast growing bacteria capable of
growth at high substrate levels and reduced pH will
dominate. A shift to slower growing scavenging
bacteria that grow better at higher pH will occur
towards the end of the reactor.
Various techniques have been applied to describe
the population dynamics within the ABR, and the
results are summarised in Table 11. By far the most
common observation involved the shift in popu-
lation of the two acetoclastic methanogens
Methanosarcina sp. and Methanosaeta sp. At high
acetate concentrations Methanosarcina outgrows
Methanosaeta due to faster growth kinetics (dou-
bling time 1.5 d compared with 4 d for
Methanosaeta), however, at low concentrations
Methanosaeta is dominant due to its scavenging
capability (Ks=30 mg/l compared with 400 mg/l for
Methanosarcina (Gujer and Zehnder, 1983)).
Tilche and Yang (1987) and Yang et al. (1988)
compared the performance and bacterial popu-
lations of an anaerobic ®lter and a Hybridised
Ba‚ed Reactor (HABR) at pilot scale treating mol-
asses wastewater with maximum loading rates of
10.5 and 5.5 kg COD/m3
d for the anaerobic ®lter
and HABR respectively. The major ®ndings of the
study were: a large concentration of Methanosarcina
at the front of the ba‚ed reactor which shifted to
Methanosaeta towards the rear, compared with a
domination of Methanosaeta in the ®lter reactor,
and, hydrogen scavenging Methanobacterium were
observed at the front of the ba‚ed reactor using
epi¯uorescence microscopy.
Explanations were o€ered to describe the lack of
Methanosarcina in the ®lter reactor. Firstly, the
acetate loading in the ®rst chamber of the HABR
was 1000 mg/l which might be close to the apparent
Ks value for Methanosarcina (data not given) and
therefore may have favoured its growth. In con-
trast, acetate levels were 10 times lower in the ®lter
reactor and therefore Methanosaeta had a kinetic
advantage and dominated in the reactor. Secondly,
lower super®cial gas production rates in the ba‚ed
reactor (5 m/d in the ®rst compartment of the
HABR compared with 9 m/d in the ®lter) resulted
in lower gas turbulence, and therefore fewer wash-
outs of bio¯ocs compared with the anaerobic ®lter.
Hydrogen levels were also measured, and the high-
Table 11. Bacterial observations in the ABR
No. Observations Technique Ref.
1 Methanosarcina predominant at front
of reactor with Methanosaeta found
towards rear
SEM, TEM, LLM Boopathy and Tilche, 1991, 1992;
Tilche and Yang, 1987; Garuti et al.,
1992; Yang et al., 1988
2 active methanogenic fraction within
biomass highest at front of reactor and
lowest in last chamber
ATA Bachmann et al., 1985; Orozco, 1988
3 bacteria resembling Propionibacterium,
Syntrophobacter and
Methanobrevibacter found in close
proximity within granules
TEM Grobicki, 1989
Methanosaeta and colonies of
Syntrophomonas also observed
4 large numbers of Methanobacterium at
front of ABR along with
Methanosarcina covered granules;
subsequent chambers consisted of
Methanosaeta coated ¯ocs
EP Tilche and Yang, 1987
5 virtually all biomass activity (>85%)
occurred in the bottom third of each
compartment where biomass was
concentrated; highest activity (92%)
found in bottom of ®rst chamber
ATPA Xing et al., 1991
6 mainly Methanosaeta observed with
some cocci; no Methanosarcina
observed
SEM Polprasert et al., 1992
7 irregular granules with gas vents
covered by single rod shaped bacteria;
no predominant species observed
SEM Holt et al., 1997
8 bacteria resembling
Methanobrevibacter, Methanococcus,
and Desulfovibrio found
ATPA, SEM, EP Boopathy and Tilche, 1992
9 wide variety of bacteria observed at
front of reactor
SEM, TEM Boopathy and Tilche, 1991; Barber
and Stuckey, 1997
Abbreviations: ATA = anaerobic toxicity assay, ATPA = ATP analysis, EP = (phase contrast) epi¯uorescence microscopy,
LLM = light level microscopy, SEM = scanning electron microscopy, TEM = transmission electron microscopy.

est concentrations (4Â 10À4
atm) were noted in the
®rst chamber of the ba‚ed reactor, and this may
explain the presence of Methanobacterium. The
results were subsequently supported by Polprasert
et al. (1992) where acetate concentrations as low as
20 mg/l enabled the domination of Methanosaeta-
like bacteria throughout a four-compartment reac-
tor.
Biomass activity
Tilche and Yang (1987) and Yang et al. (1988)
also discovered that 70% of all methane produced
in the HABR came from the ®rst compartment,
despite having only 10% of the VSS present within
the reactor, and these ®ndings supported previous
work (Bachmann et al., 1985; Orozco, 1988).
Bachmann used a procedure based on the
Anaerobic Toxicity Assay (ATA, Owen et al.
(1979)) and discovered that the active fraction of
acetate utilising methanogens as a percentage of the
total VSS varied from 5.7 to 1.8%, with the largest
values obtained at the front of the reactor and the
lowest at the rear. In a study involving an 11-com-
partment open top ba‚ed reactor treating 500 mg/l
sucrose at low temperature (13±168C), Orozco
(1988) quoted activities of 1.43 g COD-CH4/m3
in
the ®rst seven chambers and 0.72 in chambers 7 to
11.
Xing et al. (1991), and Boopathy and Tilche
(1992) used ATP analysis to determine the relative
position of the most active bacteria. Samples were
taken from the top, middle and bottom of all three
chambers from a reactor with a working volume of
150 l treating molasses wastewater at a loading rate
of 20 kg COD/m3
d. The results showed that at
least 85% of the activity came from the bottom of
each compartment with the highest activity (92%)
measured at the base of the ®rst compartment.
However, the opposite trend was found in a study
treating slaughterhouse wastewater (Polprasert et
al., 1992). The reasons for this may lie in the con-
centration of intermediates, especially acetate, at
the front of the reactor. In studies where methane
activity was higher in the front compartments
(Bachmann et al., 1985; Tilche and Yang, 1987; and
Yang et al., 1988), acetate concentrations were rela-
tively high and therefore provided the best environ-
mental conditions for the growth of Methanosarcina
which can grow quickly and eciently even at pH
values as low as 6 (Speece, 1996). Another source
of methane would be from hydrogen scavenging
bacteria such as Methanobacterium (Tilche and
Yang, 1987) and Methanobrevibacter, which would
be stimulated by the higher hydrogen concen-
trations; the net e€ect would be a high methano-
genic activity. In contrast, with dilute wastewaters,
where acetate levels are low in the front compart-
ment (as in the work by Polprasertet al), the likely
scenario is that Methanosaeta would dominate.
However, this species grows at a far slower rate
compared to Methanosarcina and is also far more
sensitive to environmental conditions such as a
reduced pH. This would encourage the growth of
acid producing bacteria that would inevitably lead
to a reduction in methane potential.
Hassouna and Stuckey (1998) showed a shift in
the activity of acid producing bacteria down the
length of an eight-compartment ba‚ed reactor.
Using the method of Owen et al. (1979), aliquots
were removed from each compartment of ABRs
treating a range of substrate concentrations. In the
foremost compartments a glucose spike was readily
converted to volatile acids within a few hours and
this contrasted with the results from subsequent
compartments which showed virtually no degra-
dation of the spike.
Granulation (and ¯oc sizes)
Although granulation is not necessary in the
ABR for optimum performance, unlike suspended
systems such as the UASB, various reports have
noted the appearance of granules in the reactor.
Boopathy and Tilche (1991) started up HABRs (the
inoculum contained 4.01 g VSS/l) with a low initial
loading rate (0.97 kg COD/kg VSS d) and liquid
up¯ow velocities below 0.46 m/h, in order to encou-
rage the growth of ¯occulent and granular biomass.
Subsequently, stable granules of 0.5 mm appeared
after one month in all chambers of the reactor and
they were reported to be growing although no data
was given; microscope studies subsequently showed
that the granules were comprised primarily of acet-
oclastic methanogens. Similarly, Tilche and Yang
(1987) found Methanosarcina coated ¯ocs held
together by ®brous bacteria resembling
Methanosaeta. The ¯ocs, which were formed after
one month, were small with diameters less than
1.5 mm and were weak. Under the same loading
conditions the authors also found densely packed
granules typical of a UASB (d < 3 mm) formed
after 3 months in an anaerobic ®lter.
Boopathy and Tilche (1992) noticed similar par-
ticles of both types described above, which grew
from 0.5 mm after one month to 3.5 mm after three
months in a hybrid reactor. Granules, which were
made from Methanosarcina clusters, were of low
density and full of gas cavities and therefore lifted
to the surface of the reactor due to high gas and
liquid velocities during high loading. The particle
size appeared to be partially dependent on substrate
type. There was little di€erence in particle size
throughout the reactor when molasses alcohol stil-
lage wastewater was treated. However, two weeks
after the substrate was altered to raw molasses with
a ten-fold increase in inlet COD a pro®le emerged
which showed a steady decrease in particle size
down the reactor. In addition, the sludge weight
increased from <600 to 900 g in the ®rst compart-
ment within the same time period (Xing et al.,
1991). Orozco (1988) reported a similar decrease in

granule size from 5.4 mm in the ®rst chamber down
to 1.5 mm in the last chamber of a reactor treating
dilute carbohydrate waste. However, on a labora-
tory scale, (Barber and Stuckey, 1997) ¯oc size
seemed to grow to a maximum near the centre of
an eight-compartment reactor and then reduce
towards the rear. Typical ¯oc sizes were 100, 230
and 175 mm in the front, middle and rear compart-
ments respectively. These authors postulated that
the ¯oc size was a function of both gas production
and COD concentration, with the largest particles
growing when COD concentrations were suciently
high to support growth, and gas production low
enough to avoid ¯oc breakage.
Solids retention capability
By using a chromic oxide sesqui tracer in a high
solids swine wastewater (51 g/l), Boopathy and
Sievers (1991) managed to measure the solids reten-
tion time for two hybrid reactors running at a
hydraulic retention time of 15 d. A three-compart-
ment reactor resulted in a solids retention time of
25 d compared with 22 d for a two-compartment
unit. The two-compartment reactor had a larger in-
itial chamber, and this provided a natural ®ltering
action that enabled it to lose fewer solids to the
e‚uent. Despite this, the three-compartment reactor
was found to be more ecient at converting the
trapped material into methane on the basis of cellu-
lose, lipid and protein measurements.
In a comparative study, Orozco (1988) calculated
the minimum solids retention time required to
achieve certain removal eciencies in ba‚ed and
UASB reactors under the same loading conditions,
and concluded that the solid residence time in the
UASB would have to be approximately 40% higher
than the ABR in order to achieve the same removal
rate. By assuming a series of perfectly mixed reac-
tors, Grobicki and Stuckey (1991), calculated the
solids retention times, biomass yield, and washout
of biomass under several experimental conditions.
Solids retention times varied from 7 to over 700 d
(5 < 80 h) and large deviations in the results were
attributed to varying degrees of granulation.
Although a strong correlation was found to exist
between the solids retention time and HRT, the
authors suggested that caution should be exercised
when using the calculated ®gures due to the
assumptions of perfectly mixed behaviour. Solids
retention times of 300 d were reported by Garuti et
al. (1992) using a 350 l reactor with a 15 h retention
time and this ®gure is far higher than those calcu-
lated by Grobicki and Stuckey (1991) under similar
conditions. These authors also calculated from the-
ory and a mass balance, that the observed yields
were very low (approximately 0.03 kg VSS/kg
COD), which implies constant biomass concen-
tration pro®les over time, but these ®ndings are in
contrast to other researchers (Boopathy and Tilche,
1991; Xing et al, 1991).
Boopathy et al. (1988) discovered that increasing
the loading rate from 2.2 to 3.5 kg COD/m3
d made
no signi®cant di€erence to the amount of solids lost
to the e‚uent, with a maximum of 0.5 g/l occurring
during start-up. These results were further sup-
ported in a hybrid reactor (Boopathy and Tilche,
1991) where virtually negligible e‚uent VSS was
found with loading rates between 6 and 12.5 kg
COD/m3
d. However, a linear increase up to 17 g
VSS/l at high loading rates (28 kg COD/m3
d) was
observed. A similar correlation was also found to
exist between the Sludge Volume Index (SVI) and
the total solids lost from a pilot scale reactor
(Garuti et al., 1992). Finally, in a recent study,
Barber and Stuckey (1997) found that twice as
many solids were lost during start-up by a reactor
running at a low HRT of 20 h compared with one
which was run on the same feed at long retention
times (80 4 40 4 20 h), and this was linked to in-
ferior COD removal since biomass accumulated fas-
ter in the reactor run at longer retention times.
MODELLING
Bachmann et al. (1983) found similar treatment
behaviour under identical conditions in an ABR,
anaerobic ®lter and a rotating biological disc reac-
tor. In order to predict reactor performance, an
attempt was made to develop a uni®ed model for
the ®xed ®lm reactors and also for the ABR. The
authors considered the sludge particles found within
the sludge bed of the ABR to be ¯uidised spheres
Table 12. Model equations for ABR systems
No. Substrate model equations Ref.
1 dS/dt = À aCSq
+QS0ÀQS, S = S0À(a/Q)CSq
Bachmann et al., 1983
2 Df(d2
Sf/dz2
) = (kSfXf)/(Ks+Sf) Bachmann et al., 1985
3a Sn=S0/[(1 + k1W1/Q)(1 + k2W2/Q)F F F(1 + knWn/Q)] Xing et al., 1991
3b Sn=[S0(1 + R)n À 1
]/[(1 + R + k1W1/Q)(1 + R + k2W2/Q)F F F(1 + R + knWn/Q) À (1 + R)n À 1
R] Xing et al., 1991
4 Df[(d2
Sf/dr2
) + (2/r)(dSf/dr)] = (kXfSf)/(Ks+Sf) Nachaiyasit, 1995
Nomenclature: a = surface area per unit reactor volume (LÀ1
), C = variable-order reaction coecient, Df=molecular di€usivity in bio-
®lm (L2
tÀ1
), k = maximum speci®c rate of substrate utilisation (MsMxtÀ1
), Ks=half-velocity constant (MLÀ3
), Q = speci®c ¯ow rate
(TÀ1
), q = variable-order reaction order, r = radius of a three-dimensional spherical particle (L), R = recycle ratio, S = substrate concen-
tration (MLÀ3
), S0=in¯uent concentration (MLÀ3
), Sf=substrate concentration in bio®lm (MLÀ3
), Sn=e‚uent substrate concentration
(MLÀ3
), W = mass of sludge = volume/[Xf] (M), Xf=bacterial density (MLÀ3
), z = distance normal to bio®lm surface (L). Numerical
subscripts refer to compartment number.

with a surface area through which the solute must
di€use for bacterial consumption. Therefore, they
used a combination of a ®xed ®lm model
(Williamson and McCarty, 1976) along with a vari-
able order model (Rittmann and McCarty, 1978)
which incorporated the concepts of liquid-layer
mass transfer, Monod kinetics, and molecular di€u-
sion to accurately describe the process (Table 12).
Two di€erent approaches were employed; the ®rst
was based on the concept of a rate limiting sub-
strate (assumed to be acetate and propionate) dif-
fusing into a ``deep'' ®xed bacterial ®lm.
Application of the model was made possible by esti-
mating the speci®c surface area in each of the reac-
tor chambers from a data set, and then applying
the results to simulate behaviour at di€erent load-
ings. Although initial predictions were good, the
model underestimated the level of COD removal at
higher loading rates. The reasons for the discrepan-
cies were given to be an unrealistic assumption of a
constant di€usion layer depth which would decrease
at higher loading rates due to increased gas pro-
duction, thereby improving substrate/biomass con-
tact and ultimately reactor performance.
The second evaluation was made by assuming a
series of completely mixed dispersed growth reac-
tors using Monod kinetics. Here values of the active
micro-organism concentration were determined
within each compartment for one loading, and the
data applied to the same loading rates as with the
``®xed ®lm'' model. The results of the second model
termed ``the dispersed growth model'', did not give
a realistic interpretation of the data since di€usional
limitations were not considered.
Further work using the ®xed ®lm model was car-
ried out by Bachmann et all. (1985) on ba‚ed reac-
tors with an in¯uent substrate concentration of 8 g
COD/l. The model predicted the following beha-
viour: a decrease in treatment eciency with (a)
decreasing in¯uent substrate concentration at con-
stant loading rates, (b) an increase in organic load-
ing at constant in¯uent substrate concentration, and
(c) an increase in recycle ratio at constant HRT
since the reactor approaches completely mixed
behaviour. Reactor eciency improved with redu-
cing substrate concentration at constant HRT.
Some of these ®ndings were mirrored in the work
of Xing and Tilche (1992) on the modelling of a
hybridised form of the ba‚ed reactor which had a
working volume of 150 l, and treated 20 kg COD/
m3
d of molasses wastewater. The model focused on
the ®ndings of ATP testing which concluded that
virtually all of the active biomass was held within
the base of each compartment, so the biomass
weight and not concentration was used in the
model. The main assumptions of the model were:
all substrate consumption occurred within a granu-
lar sludge bed, and, the sludge bed was perfectly
mixed due to gas evolution. The following predic-
tions were made from the model: at constant or-
ganic loading the treatment eciency increased with
increasing in¯uent substrate concentration; as HRT
was reduced the performance of the reactor
decreased; performance deteriorated with increasing
loading (11±16 kg COD/m3
d) with a constant
sludge weight; an improvement in COD removal
eciency was observed with increasing sludge
weight until a certain concentration was reached,
above which reactor performance becomes indepen-
dent of biomass concentration; and ®nally an
increase in recycle ratio coincided with a subsequent
decrease in COD removal.
Bachmann et al. (1983, 1985) assumed that the
¯oc diameter was very large relative to the active
bio®lm depth. However, this seems to be an unwar-
ranted assumption since in anaerobic bio®lms the
electron donor and acceptor are often the same or-
ganic, and since the three main microbial groups
are symbiotic, ¯oc particles may have active cores
even with 3 mm diameters. In addition, many ®lm
supports are not perfectly ¯at, but can be con-
sidered suciently ¯at if the biologically active
thickness of the bio®lm is less than about 1% of
the radius of curvature (i.e. the radius of a sphere
plus a di€usion layer, Rittmann and McCarty
(1978)). At high loading rates this is not the case in
the ABR, since sludge particles within the reactor
act as ¯uidised spheres with a surface area through
which the substrate must di€use for consumption
(Bachmann et al., 1985). These facts imply that a
spherical model would provide a better ®t than a
simple planar one.
Nachaiyasit (1995) derived a spherical model
using Monod kinetics combined with molecular dif-
fusion of the substrate into the biomass aggregates
based on the assumptions that: (i) substrate concen-
tration could be described by a single parameter,
COD, (ii) biomass concentration can be adequately
described by a single parameter, VSS, (iii) the bio-
mass composition is constant during balanced
growth and (iv) the biological reactions of import-
ance occur at constant temperature and pH. The
calculation of important model parameters such as
di€usion layer thickness, and liquid phase mass
transfer coecient followed the techniques pro-
posed by Bachmann et al. (1985). In general, the
model predicted better COD removal than was ex-
perimentally measured, and was most accurate for
high loading rates (8 and 15 g COD/l at 20 h) than
at short retention times (10, 5 h HRT with feed
concentration of 4 g COD/l), but showed large devi-
ations for the ®rst couple of compartments for
some of the simulations. As with previous models
certain trends appeared with the results, namely a
decrease in removal eciency with increasing re-
cycle ratios, decreasing HRTs (with ®xed substrate
concentration), and increasing substrate concen-
trations (with ®xed HRT). However, the spherical
model did provide a closer ®t than the earlier pla-
nar ®xed ®lm equations. Based on the ®ndings of a

sensitivity analysis that showed ¯oc surface area
and ¯owrate had the greatest in¯uence on model
predictions, the model was modi®ed by making the
surface area a ®tting parameter. Nachaiyasit then
compared the results obtained from di€erent models
based on the same assumptions and with the same
experimental data, and found the closest ®t with the
spherical model. It was concluded that while the
predictive capacity of the spherical model was not
always good, it was useful as a tool for understand-
ing the interaction between the various system par-
ameters, and therefore could be used as a basis for
the development of better predictive models.
It seems that a combination of theoretical con-
siderations and experimental ®ndings can be used
together in order to generate models with a more
realistic ®t. Since the accuracy of any model
depends critically on the wastewater and substrate
used, kinetic data should be experimentally deter-
mined for each compartment once the reactor is at
steady state (Bachmann et al., 1985) by using simple
bioassays. Such an approach may have enabled
Nachaiyasit's spherical model to give a more realis-
tic ®t at the front of the reactor. All modelling so
far has used acetoclastic methane production as the
rate-limiting step. However, it is evident from
Section 5, that the structure of the reactor will
cause a shift in the population dynamics of the two
species (Methanosarcina and Methanosaeta) respon-
sible for acetate consumption. Since both archea
di€er widely in kinetic ability, acetate loadings and
pH will have an e€ect on reactor performance in
each compartment. For reactors treating medium to
high strength wastes acetate consumption at the
front of the reactor will be higher than for a low
strength waste. This will result in most of the COD
being removed in the front of the reactor. For low
strength wastes, acetate loadings will be low and
this will encourage growth of Methanosaeta with lit-
tle COD removed in the acidi®cation zone.
Channelling has been shown to be an important
phenomenon in the ABR (see Section 3) and will
a€ect the accuracy of any model. In order to take
channelling into account it is necessary to calculate
the number N of ideally mixed reactors in series
using tracer studies (see Section 3.1). The results of
these experiments could then be input as the num-
ber of real compartments into a reactor in series
model. Also, correlations are available which show
the e€ect of hydrodynamic dispersion on the sub-
strate di€usion coecient (Bear, 1972).
Furthermore, by calculating the minimum solids
retention time (Orozco, 1988), it should be possible
to determine the correct compartment size for a
given treatment eciency. On the basis of the litera-
ture it seems that for most cases only 2±4 compart-
ments are necessary for adequate COD removal.
However, reactors with more compartments will be
far more resistant to hydraulic and organic shocks,
since they will protect against the shift in acid pro-
duction towards the rear. Therefore a compromise
will exist between optimal (required) compartment
number, ``safe'' compartment number, and also
up¯ow liquid velocity.
Despite the less than perfect predictive capabili-
ties of the models described above, there is an
urgent need to generate models for larger scale reac-
tors. Boopathy and Tilche (1991) pointed out that
at larger scale a greater evolution of gas per com-
partment cross sectional area can be expected, and
this would cause an increase in mixing which would
subsequently improve mass transfer rates leading to
greater eciencies, but perhaps increased solids
loss.
Finally, it is also necessary to model reactor
behaviour when hydrolysis is the rate-limiting step,
as is the case with high solid in¯uents and lipid con-
taining wastewaters, since by assuming ®rst-order
rate kinetics it is possible to calculate the minimum
solids retention time to achieve a given eciency
(Pavlostathis and Giraldo-Gomez, 1991). With
wastewaters containing a large amount of particu-
late material, it seems likely that COD removal will
be low at the front end, and that the VFA pro®le
formed will be shifted down the reactor, unless the
reactor is modi®ed, (Boopathy and Sievers, 1991) or
extra compartments added, a drop of eciency may
result.
FULL-SCALE EXPERIENCE
The performance data of a full-scale plant, treat-
ing domestic waste from a small town in Columbia
(Tenjo, population <2500 inhabitants, Orozco
(1997)), is presented in Table 13. The Tenjo reactors
were designed to give a liquid up¯ow velocity of
3 m/h based on laboratory ®ndings. However,
despite following strict guidelines for start-up, the
Table 13. Full-scale data for an anaerobically operated ba‚ed
reactor Tenjo, Colombia
Wastewater
1 Composition domestic/industrial mixa
2 Strength (g BOD5/l) 0.314
3 Total solids (g/l) 0.90
4 Volatile solids (g/l) 0.25
Performance
1 OLR (kg/m3
d) 0.85
2 Removal eciency (%) H70
3 HRT (h) 10.3
4 E‚uent BOD5 (g BOD5/l) H0.1
Reactor design
1 Reactor con®guration open top reactor
2 Reactor number 2
3 Reactor volume (m3
) 394 (197 each)
4 Reactor dimensionsb
(m) 2.7:17:4.3
5 Compartment number 8
6 Liquid up¯ow velocity (m/h) 3.00
7 Packing material plastic boxes
8 Settling chamber internal gas/solid separation
Economics
1 See text
Miscellaneous
1 Temperature (8C) 15
a
Industrial dairy waste.b
Reactor dimensions: height:length:width.

reactors experienced several practical problems
during early operation. Hydraulic shocks increased
solids washout, and poor screening of solid material
caused the plastic packing media to ¯oat with gas
production. These problems were overcome by
using a by-pass pipe during the rainy season and
improved screening facilities. The reactors per-
formed well with approximately 70% COD re-
duction and 80% removal of suspended solids over
a two-month period. Varying the volumetric load
between 0.4 and 2 kg/m3
d had no e€ect on removal
eciency. However, the author concluded that a
polishing lagoon was necessary to achieve discharge
quality e‚uent. Work is currently being undertaken
to provide a wastewater treatment plant for a larger
town.
Although a detailed economic study was not pre-
sented, construction costs for the ba‚ed reactor
were 20% less than those for UASB reactors in
Columbia running at ambient temperature, and ®ve
times less than a conventional activated sludge
plant for a small town.
FUTURE PROSPECTS FOR THE ABR
The ABR shows promise for industrial waste-
water treatment since it can withstand severe
hydraulic and organic shock loads, intermittent
feeding, temperature changes, and tolerate certain
toxic materials due to its inherent two-phase beha-
viour. Despite comparable performance with other
well established technologies (Table 14), its future
use will depend on exploiting its structure in order
to treat wastewaters which cannot be readily trea-
ted. Outlined below is a list of possible processes
that are feasible in the ABR.
In situ aerobic polishing
Unpublished work in this laboratory has shown
that an aerobic polishing step can be inserted within
an ABR with no detrimental e€ect on reactor per-
formance. This is due to the fact that ``aerophobic''
methanogens can remain active even when oxygen
is present, and whilst inside immobilised aggregates
methanogenic archea are well shielded from oxygen
by layers of facultative bacteria (Lettinga et al.,
1997). Also, processes which inherently require both
anaerobic and aerobic treatment (or detoxi®cation)
can be dealt with within a single reactor unit, such
as black hemp liquors, wood extractives, coal pro-
cessing industry, petrochemical, and textile dye
wastewaters (Lettinga, 1995) thus signi®cantly redu-
cing capital costs.
Total nitrogen removal. Work is currently being
undertaken to treat ammonia containing waste-
waters with an anaerobic/aerobic ba‚ed reactor for
total nitrogen removal. Ammonia present in the
wastewater passes through the anaerobic compart-
Table 14. Treatment eciencies for various reactor con®gurations
Feedstock Reactor
type
Reactor
volume (l)
Inlet COD
(g/l)
Loading
rate (kg/m3
d)
COD
removal (%)
Ref.
Carbohydrate ABR 6.3 7.1 1 79 Bachmann et al., 1983
ABR 6 1±10 2±20 72±99 Bae et al., 1997
ABR 75 0.44±0.47 0.96±1.66 84±93 Orozco, 1988
UASB 4.8 1±10 2±20 <50±97 Bae et al., 1997
UASB À 0.49±0.55 1±2.2 77±86 Orozco, 1988
AF 0.4a
8 1 92 Bachmann et al., 1983
USSB 4.2 60±80b
75 75±89 van Lier et al., 1996
AAFEB H0.4 0.05±0.60 0.8±4.8 40±95 Switzenbaum and Jewell, 1980
Slaughterhouse ABR 5.16 0.48±0.73 0.9±4.7 75 Polprasert et al., 1992
UASB 0.73 2.7 77 Zheng and Wu, 1985
UASB 1.50±2.20 7 85 Sayed et al., 1987
UASB 30 1.50±2.20 6±10 87±91 Lettinga et al., 1982
UASB 2 8 1±6.5 90 Ruiz et al., 1997
AF 2 8 1±6.5 <90 Ruiz et al., 1997
Molasses HABR 150 115±990 20 77 Boopathy and Tilche, 1991
HABR 150 115±990 28 50 Boopathy and Tilche, 1992
HABR 150 5±10 5.5 98 Tilche and Yang, 1987
UASB H85 100 24 75 Sanchez Riera et al., 1985
AF 125 5±10 10.5 98 Tilche and Yang, 1987
Greywater ABR 8 0.48 0.4 63±84 Witthauer and Stuckey, 1982
AF 8a
0.48 0.4 64±89 Witthauer and Stuckey, 1982
Piggery ABR 15 58.5 4±8 62±69 Boopathy and Sievers, 1991
UASB 7 5.5 2.83 60±80 Cintoli et al., 1995
Phenol ABR À 2.2±3.2 1.67±2.5 83±94 Holt et al., 1997
UASB 0.9c
98 Zhou and Fang, 1997
UASB 5.2 2 90 Chang et al., 1995
AF 69,000 H1.8 5.67 54 Kanekar et al., 1996
Sulphated
ABR 10 20 20 50 Fox and Venkatasubbiah, 1996
2-phase 2.7 (2.5)e
45.2 À 85 Reis et al., 1995
UASB 5.75 0.7±2 H5 90±95 Visser et al., 1992
AF 1 49.8 11±18.6 29±36 Hilton and Archer, 1988
a
Liquid volume.b
Sucrose with VFA mixture, thermophilic treatment.c
Phenol concentration.d
COD:SO4 ratio 8:1 for ABR, 10:1 for 2-
phase, 2:1 4 0.5:1 for UASB (thermophilic), 8:1 4 4:1 for AF.e
Acidogenic stage made up of two reactors with a total volume of 2.7 l.
The number in parentheses refers to a single methanogenic stage.

ments largely unmetabolised, and is then oxidised
to form nitrates and nitrites at the rear of the reac-
tor. These can then be recycled to the anaerobic
section where they act as alternative electron accep-
tors and are reduced to nitrogen.
Complete sulphur removal. Sulphate is reduced at
higher redox potentials than that at which methano-
genesis begins (Henze and HarremoeÈ s, 1983), and
will therefore be converted to hydrogen sulphide at
the front of a ba‚ed reactor at the expense of
methanogenesis (Fox and Venkatasubbiah, 1996).
Micro-aerobic polishing could be achieved within
an aerobic stage to produce elemental sulphur,
which could be recovered eliminating the need of a
separate trickling ®lter unit.
Enhancement of two-phase properties (better pH and
temperature control)
The optimum pH for a two-phase system has
been widely quoted to be approximately 5 (Ghosh
et al., 1975; Aivasidis et al., 1988; Speece, 1996).
This implies that less bu€ering would be required in
a ba‚ed reactor since the pH is routinely above 6
in the ®rst compartment. Alternatively, bu€ering
and/or nutrients could be added separately in latter
compartments to provide optimal conditions for
scavenging methanogens.
CONCLUSIONS AND RECOMMENDATIONS
Laboratory, pilot and full-scale work has shown
that the ABR is capable of treating a variety of
wastewaters of varying strength (0.45 < 1000 g/l),
over a large range of loading rates (0.4 < 28 kg/
m3
d), and with high solids concentrations with sat-
isfactory results (Table 6). Long biomass retention
times are possible without granulation and solids/
liquid separation devices, and a selective pressure
exists which enhances the development of appropri-
ate bacterial populations in various parts of the
reactor. This reactor con®guration confers consider-
able resistance to toxic materials, shields syntrophic
bacteria from elevated hydrogen levels, and results
in high removal eciencies even at low hydraulic
retention times (2±6 h). The physical structure of
the reactor allows various modi®cations to be
made, such as an in situ aerobic polishing stage,
resulting in providing the capability to treat waste-
waters that currently require at least two separate
units, therefore substantially reducing capital costs.
However, in order to enhance the commercial po-
tential of the ABR, more work still remains to be
done in the following areas: modelling the fate of
SMPs, solids, intermediate products, and COD
removal; nutrient requirements; treatment of toxic
wastewaters (e.g. polychlorinated aliphatics, nitrated
organics, xenobiotics, haloaromatics, surfactants)
which have been treated with success anaerobically;
and an improved understanding of the factors con-
trolling bacterial ecology. Finally, Table 15 shows a
list of recommendations based on this review of the
literature.
AcknowledgementÐThe authors would like to thank
Professor Chynoweth for his generous help with providing
material for this review and the BBSRC for ®nancial sup-
port.
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